Help ~ Preferences ~ Deauthorize.
But be aware that you will no longer be able to read books bought/borrowed under the old id/account.
If you have old books you still want, you must recover access to the old account ... once you have done that you can change the email associated with it.
If you still have access to the old email, you can recover the password for the old account from the Adobe singin page.
Otherwise, you must contact Adobe and see if they can help recover your access to the old account